Adding Shell plugin, but with the requirement that ENABLE_SHELL_PLUGIN must be defined and true, to give sysadmins some control. Closes #45.

PHPCI/Plugin/Shell.php

@@ -41,6 +41,10 @@ class Shell implements \PHPCI\Plugin
     */
     public function execute()
     {
+        if (!defined('ENABLE_SHELL_PLUGIN') || !ENABLE_SHELL_PLUGIN) {
+            throw new \Exception('The shell plugin is not enabled.');
+        }
+        
         $success = $this->phpci->executeCommand($this->command);
         
         return $success;